When controlling current to a motor, a solenoid or other inductive load, it is frequently desired to control the current by rapidly switching the current on and off so that the average current meets a desired goal or command value. The current switching is generally accomplished by one or more switches in series with the load and the power supply. When the current is switched on it increases at a rate limited by the inductive reactance of the load. When the current is switched off it slowly decays at a rate also determined by the inductive reactance. During the current decay period a path must be provided for the current, known as induced or recirculation current.
A common motor driver circuit is an H-bridge which has two arms connected from opposite sides of the motor to the power source and two more arms connected from opposite sides of the motor to ground. Each arm contains a switch such as a power MOSFET so that by selective switch control the motor can be driven in either direction by current flowing from the power source and through the motor to ground. It is convenient to use this type of driver in conjunction with a driver interface when using a microcomputer to control the motor. As is well known, load current feedback is often desirable for comparison to the command value to achieve closed loop control. The recirculation current as well as the applied current is required to be represented by the feedback. In the case of the H-bridge configuration, the motor current is difficult to sense because it is bidirectional and a sensor would yield negative voltages at times and much circuit complexity would be required to accommodate it.